The present invention relates to a process for measuring electric charge quantity flowing per unit time through a vacuum volume range in a given direction, to a measuring apparatus for carrying out that process in which particle currents are measured simultaneously in a weighted manner, and to a use of the process and the measuring apparatus such that the collectors are operatively connected simultaneously with current measuring circuits for different current ranges, and a superimposing unit is operatively connected with the current measuring circuits such that output signals of the current measuring circuits are superimposed.
It is known to at least partially collect the electric charge quantity flowing in a vacuum, whether caused by ions or electrons, by providing collector arrangements in the flow path. The collector arrangement is connected to a current measuring circuit for a galvanic current, that is, an electron conduction current, and the galvanic current caused by the collecting rate is measured by a current measuring arrangement, such as an electrometer circuit. Such collector arrangements are also connected in front of photoelectron multipliers in order to increase the sensitivity.
The current measuring range is limited by the dynamic range of the collector arrangement or by the dynamic range of the operability of the current measuring arrangement or by the electronic system provided for this purpose. In practice, the dynamic range is increased by the change-over of the sensitivity, i.e., of the measuring range, of the electronic measuring system, and as a result of the fact that, with respect to the flow direction of the charge carriers in the vacuum, several collector arrangements are arranged in parallel (side by side), the dynamic range is in each case connected with an electronic measuring system with a different measuring range.
It is also known to use logarithmic amplifiers for measuring the galvanic current which, however, in low current ranges (i.e., for example, in the pA-range), is accompanied by a loss of accuracy and results in large measuring time constants during the measuring of galvanic currents, such as below 10 pA. "Accuracy" as used herein is defined as a measure of the closeness of agreement of an instrument reading compared to that of a primary standard having absolute traceability to a standard sanctioned by a recognized standards organization. This definition corresponds to the one used in "Low Level Measurements", Keithley Instruments, Inc., Cleveland, Ohio 44139, USA (4th Ed.). Therefore, although logarithmic amplifiers would result in a large dynamic range, they are unsuitable for this measurement in many practical applications.
By positioning the collectors in parallel in the particle current, for example, in an ion or electron beam, implemented in the above-mentioned manner, generally two objectives are contemplated. First, the dynamic range can be expanded in that the parallel systems are each equipped with electronic current measuring systems of different sensitivities. Second, the quality of the measurement can be evaluated to the extent dependent on the collectors, in that measuring signal deviations below the parallel collector stages can be utilized for signal correcting purposes. That is, the same readings occur in the case of the same particle currents. In this case, for calibrating purposes, a particle current is switched over between the parallel collectors.
Known systems, also called Faraday collector systems, particularly for measuring ion and/or electron currents, or combinations of such Faraday collector systems with photoelectron multipliers, such as the MCP (micro channel plate), CT (channeltron), ST (spiraltron), and electron multipliers with a discrete dynode number are systems which are operated in parallel or time-sequential systems which can be changed over with respect to time.
Parallel measuring systems of collectors are found in multiple applications, such as, for example, in magnetic mass spectrometers, such as the MAT 261 of Finnigan Co.; the leak indicator ASM 120 of Alcatel Co. or in the M.T.Esat apparatus, a 61 cm multi-detector mass spectrometer at the Australian National University introduced in Nucl. Instrum. & Methods, Physical Research Section B.
DE-OS 31 39 975 describes the sequential calibration of such instruments with known reference quantities. The collector arrangements per se are not affected by such calibrations, but only the electronic current measuring system connected therebehind.
Time-sequential measuring systems which, for example, have the configurations such as Faraday collecting system/Faraday collecting system, Faraday collecting system/MCP, Faraday collecting system/CT, Faraday collecting system/photoelectron multiplier (SEV), Faraday collecting system/ST, are operated in a time staggered manner, for example, by the change-over of the particle beam as described, for example, in DE-37 20 161 and in French Patent FR-2 600 416-A.
The basic disadvantage of providing collectors arranged in parallel with respect to the particle current is the fact that the charge quantity is measured at different locations. The origin of a detected signal change on one of the collectors provided in parallel in comparison to the others must not necessarily be the result of a faulty adjustment of the assigned electronic measuring system. It may also be the indication of a changing current density distribution or a change of the flowing charge density distribution above the observed surface of the particle current.
These effects, like the changes of the characteristics of the collector itself, all result in a signal change at the electronic current measuring system and cannot be separated or distinguished from one another. In fact, the parallel connection of collectors is frequently used for specifically measuring the charge density distribution in the current as a function of location. Reference is made in this respect to the above-mentioned Nucl. Instrum. & Methods, Phys. Res. Sect. B article, as well as to T. A. Peyser, et al., "Segmented Concentric Faraday Cup for Measurement of Time-Dependent Relativistic Electron Beam Profiles", Rev. Sci. Instrum. 62(12), and K. Asano, et al., "Multi-Faraday-Cup-Type-Beam Profile Monitoring System for a Dual-Beam Irradiation Facility," Nucl. Instrum. & Methods, Phys. Res. Sect. B.
Time-sequential processes in the case of which, particularly for reasons of selecting the measuring range, a change-over is necessary, are suitable only to a limited extent for the measuring of rapidly changing currents because, during the change-over operations, the measuring system will be blind and partly, together with the change-over, a change of the ion or electron optics will be required in the collector range.
EP-A-0 172 477 describes a process in which, particularly also ion-optically, when a given particle current limit value is reached, particles are extracted in a targeted manner. This also is a time-sequential process with the above-mentioned disadvantages.
In the case of a glow cathode ionization manometer, it is also known from DE-OS 28 36 671 to exclusively measure high pressures of the gas absorber by the simultaneous measuring of the current on an ion collector and on a lattice electrode in that the measured currents are mathematically combined and a conclusion is drawn concerning the pressure and the absorber.
The use of cascaded secondary-electron emitter grids for measuring the beam profile on a proton accelerator is known from "Small Pickup for Current and Profile of Beam of Pulsed Electrostatic Proton Accelerator", V. N. Getmanov, Instruments and Experimental Techniques, Volume 28, No. 1, February 1985, New York, U.S.A.
S. Paszti, et al. "Current Measurement and MeV Energy Ion Beams", Nuclear Instruments & Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, vol. B47, No. 2, Apr. 1, 1990, Amsterdam, discusses measurement of an ion or electron beam by a mechanically choppered Faraday.
It is an object of the present invention to provide a process and a measuring apparatus which eliminate the above-mentioned disadvantages and which make it possible continuously to measure currents of charged particles with high accuracy with a significantly increased measuring range.
The foregoing object has been achieved according to the present invention by collecting the charged particles in a given direction simultaneously and in a cascaded manner.
As a result of the fact that the collecting of the particles is carried out in a locally cascaded manner, thus locally in series, in contrast to the previous parallel collecting approach, one advantage is achieved that the same surface range is measured on the collecting cascade with respect to the particle current in the vacuum. As a result, the best possible mutual calibration may take place of the cascaded collecting or of the collectors provided for this purpose together with the current measuring circuits connected therebehind.
In a currently preferred embodiment, the cascaded collecting is carried out selectively with respect to given energy ranges of the particles, or by the fact that relative respective particle collecting rates are given.
Furthermore, the selectivity is advantageously established or adjusted by applying predeterminable electrostatic potentials to the cascaded collector stages and/or by providing the optical transmission at the provided collector stages. Optical transmission, in this case, is the interstice-to-solid ratio which the approaching particles encounter at a collector arrangement, such as a collecting grid.
A measuring apparatus according to the present invention measures the particle currents simultaneously and process them preferably in a weighted manner. The process and measuring arrangement according to the present invention are used particularly for partial-pressure and total-pressure measuring apparatuses, such as, for example, a mass spectrometer or Bayard-d'Alpert tubes.